One of the principal advantages of plastics, their lack of conductivity, has become a major disadvantage for certain applications. Because plastics, which are extensively used in the construction, housing and packaging of electronic equipment, are insulators, they do not shield such equipment from electromagnetic/radio frequency interference ("EMI/RFI") from outside sources. Stray electromagnetic signals can adversely affect sensitive electronic equipment such as computers, navigational instruments, process controls, communication equipment and the like.
In an effort to prepare a truly effective EMI/RFI shielding material, a variety of finely divided conductive substances have been dispersed in plastics to render them conductive. A representative list of such materials includes aluminum flakes, silver flakes, metal-coated carbon fibers, stainless steel fibers, carbon black, nickel-coated graphite, silver-coated copper and the like.
The amount of a given conductive material which can be dispersed into a plastic resin to form an EMI/RFI shielding material is usually limited by the effect thereof on the melt viscosity of the filled resin and other factors such as workability of the material, brittleness and adhesiveness of coatings or sheets made therefrom, and the like. Melt viscosity is a primary criterion for the usefulness of a composition as a shielding material.
Melt viscosity is critical because, in many instances, shielding material must be moldable. Incorporating particulate or finely divided conductive substances into thermoplastic resins can increase the melt viscosity to where they will not fill the mold. Raising the molding temperature to compensate for this is only effective to the point where thermal degradation of the polymer must be considered. While a comparatively greater amount of conductive material can be incorporated into certain thermoset substances such as rubber, the poor qualities of the resulting composition in terms of workability make them substantially less desirable than filled thermoplastic resins such as poly(vinyl chloride).
A major disadvantage of the EMI/RFI shielding compositions known at present is that they suffer a loss of effectiveness if there is a gap or opening in their protective coating. With a metal coating, for example, a small slit or crack in the coating will cause the shielding value to drop to almost zero. In other words, they are effective if they completely enclose an electronic device. This, of course, is usually not possible because of the design of the device or its housing. Even overlapping sheets of conductive shielding is not effective in preventing leakage of radiation which escapes at the edge of the sheets. It may be necessary to provide a gasket at the interface of two shielding sheets, such as is described by Bogner in U.S. Pat. Nos. 4,396,795 and 4,414,425 because radiation, even though it can't go through sheets of such material, will go around them.
In accordance with this invention, a composition is provided which is very effective as EMI/RFI shielding material and which can tolerate a discontinuity in a protective coating made therefrom without total loss of effectiveness.